In recent years, secondary battery represented by a lithium ion battery has increasingly wide range of applications, mainly for in-vehicle use, and the production volume is constantly increasing. In the process of manufacturing a secondary battery, a method is often employed, in which the components of the battery are sealed in a metal case. In addition, in order to improve the space efficiency of the secondary battery, a rectangular cell case is often employed rather than a conventional circular case. In both cases where the circular cell case is employed and where the rectangular cell case is employed, laser welding, by which a structural body can be manufactured efficiently, is more often used for manufacturing lithium ion batteries.
In the manufacturing process of a lithium ion battery, laser welding is often used in the following three processes:
1. Cap seam welding for bonding an aluminum can body and a cap member together.
2. Seal welding for closing a pouring hole for pouring an electrolyte.
3. Component welding for electrically connecting a plurality of cells in parallel or in series.
However, in the seal welding of the second process among the above-mentioned processes, a laser beam is caused to penetrate from a surface of an object to be joined to an opposite surface thereof to melt an opposed object, thereby integrating the two objects. Consequently, micro cracks occur in the course of solidifying process of the objects.